<!DOCTYPE html>
<html>

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Sugar</title>
</head>

<body>
  <script>

    const htmlStr = '<img src="http://xxxx.png" /> <img src="http://xxxx.png" />'


    const res = htmlStr.replace(/<img/g, '<img class="rich_img"')

    console.log(htmlStr);
    console.log(res);

  </script>
</body>

</html>